Halobacteria can use light energy to drive essential metabolic processes, i.e., they are photosynthetic. The light-absorbing pigment is a rhodopsin-like protein bacteriorhodopsin, which occurs in their cell membrane in the form of crystalline patches known as the purple membrane. These can be isolated in pure form and their structure is known to 7 A resolution. When bacteriorhodopsin absorbs light it undergoes a photoreaction cycle and translocates protons from the interior of the cell into the medium. A substantial amount of the light energy is stored in the resulting electrochemical proton gradient. We propose to study the structure and function of this light-driven proton pump and its role in the energy metabolism of the cell. Genetic studies will concentrate on isolating the gene for bacteriorhodopsin and the production of mutant bacteriorhodopsins. The detailed mechanism of proton translocation will be explored by characterizing the intermediates in the photoreaction cycle and their kinetics using mainly spectroscopic technics. The structure will be further explored by X-ray diffraction and high resolution low dose electron microscopy. Electric and spectroscopic measurements will be carried out on model systems mainly on planar films with incorporated purple membrane. Studies of intact cells and subcellular preparations will explore the role of bacteriorhodopsin in the energy metabolism of the cell concentrating on the light-driven ATP synthesis and solute transport phenomena. Bacteriorhodopsin also acts as the sensor for a phototactic response in the cell and we shall investigate the signal transduction mechanism involved. Parallel studies will be carried out on rhodopsin and the H ion-ATPase of other bacteria, mitochondria or chloroplasts, searching for expected similarities in the light-sensing and proton-pumping mechanisms.